Radiation effects along a UV-B gradient on species composition and diversity of a shallow-water macrobenthic community in the western Baltic

Although the 'intermediate disturbance hypothesis' (IDH) was originally postulated for marine hard-bottom communities, it has rarely been tested for this community type. We experimentally examined the effects of ambient plus artificially enhanced UV-B radiation (eUV-BR) along an intensity gradient on the abundance of individual species, species composition, and diversity of a macrobenthic community in the western Baltic, Germany. Plots were either exposed each day for 0.5, 1, 2, 4, and 8 h to the maximum expected future UV-BR or were left untreated (= ambient irradiance). Species recruitment and succession on artificial substrata were followed for 5 mo. Transient unimodal patterns for species richness and diversity H' were produced along the UV-BR gradient early in succession. The density of the competitively superior mussel Mytilus edulis and the abundance of the foliose green alga Ulvopsis grevillei were positively related to UV-BR exposure; suggesting conditions different from those predicted by the IDH-generated unimodal patterns. Protective shading of substratum by U. grevillei and the resultant mitigation of the UV-BR impact for succeeding colonizers may be important in this context. Species composition differed persistently among eUV-BR treatments. In contrast to mussels and U. grevillei, the abundance of red algae was adversely affected by eUV-BR along the disturbance gradient. Our results suggest that expected future UV-BR levels will have limited influence on the diversity of shallow-water macrobenthic fouling assemblages in the western Baltic, whereas their species composition may be affected over longer periods.